The present invention is directed to a method and apparatus for controlling the amount of power that is extracted from an electrical power source, and more particularly to a system for reducing the total load imposed on a power source by individual loads that consume alternating current. A particularly useful application for the invention is to automatically drop 240 volt loads in a residence or small business, if commercial power is lost and the residence or small business switches to an on-site power source, while continuing to supply power from the on-site power source to 120 volt loads.
In the United States, utility companies usually supply their residential and small-business customers with 60 Hz AC power at two different voltages, one of these voltages having twice the magnitude of the other. The lower voltage is typically used for relatively light loads such as electric lighting and small appliances and the higher voltage is employed for heavier loads such as water heaters or air conditioners. Both the lower voltage and the higher voltage are usually expressed in terms of an effective or root-mean-square voltage since the instantaneous value of an AC voltage varies from moment to moment. The xe2x80x9ceffective voltagexe2x80x9d means the DC equivalent of an AC voltage, or more precisely the DC voltage that would generate heat in a resistive load at the same rate as the AC voltage in question. For an AC voltage having a sinusoidal waveform the effective or rms voltage is the peak voltage of the waveform divided by the square root of two. In this document, terms such as xe2x80x9cvoltagexe2x80x9d will be understood to refer to the effective voltage unless the instantaneous voltage or the peak voltage is explicitly mentioned.
Most loads found in a home or small business can operate satisfactorily over a range of voltages. A light bulb that glows brightly at 120 volts will be dimmer at 105 volts but will still produce light. A home or small business may also have voltage-sensitive loads such as electronic equipment but such loads usually include power supplies with voltage regulators, and hence they, too, can accommodate a reasonable range of voltages. This is fortunate since the power supplied by a utility company to its residential and small-business customers may vary in voltage even under normal conditions depending on a number of factors, such as the voltage at the generator, the total load imposed on the generator, and the distance between the customer and the generator. In the United States the lower voltage that is supplied by a utility company to its residential and small-business customers is usually 120 volts AC and the higher voltage is twice that, or 240 volts AC, but it is to be understood that these are nominal voltages and the precise values that are actually received by customers may vary from customer to customer and, for the same customer, from time to time. In this document, the term xe2x80x9c120 voltsxe2x80x9d means the normal household voltage for running small appliances, even if the actual effective voltage departs somewhat from this value, and similarly the term xe2x80x9c240 voltsxe2x80x9d is a nominal value.
It is an unfortunate fact of life that customers sometimes lose access to commercial power. Perhaps the power lines are broken during a storm, for example, or maybe it is necessary for the utility company""s technicians to disconnect power lines while working on them. A customer who does not want to be inconvenienced during such outages may provide his own on-site power source, which is quickly switched in during an outage.
In order to be economically feasible for most customers, the capacity of an on-site power source must be relatively modest. The limited capacity may be insufficient for all the loads that happen to be turned on when an outage occurs. This problem is particularly acute if a water heater or other 240 volt load happens to be on.
One might consider monitoring for an outage and using electrically-controlled circuit breakers to switch off 240 volt loads when a fault occurs. The problem is, though, that conventional electrically-controlled circuit breakers have an operation time of up to 100 milliseconds. For 60 Hz power, this presents a six cycle delay in response time, and this delay may be sufficient to cause circuitry associated with the on-site power source to trip off-line due to a large instantaneous drain imposed by 240 volt loads. The result would be a complete loss of power, which defeats the customer""s purpose in purchasing an on-site power source.
A primary object of the present invention is to provide a load reduction method and system which rapidly eliminate, or at least substantially reduce, 240 volt loads while maintaining 120 volt loads.
Another object is to control the effective AC voltage that is supplied to a load by varying the phase angle between AC potentials that are supplied to the load.
These and other objects that will become apparent in the ensuing detailed description can be attained by providing a load reduction method that comprises the steps of supplying AC power to a first-leg user line, supplying AC power to a second-leg user line which is normally about 180xc2x0 out-of-phase with the first-leg user line, and changing the phase angle to about 0xc2x0 in order to drop a load that is connected to the first-leg user line and the second-leg user line.